The present invention relates to variable displacement compressors varying displacement in a range from minimum to maximum and air conditioning apparatuses incorporating the compressors.
A typical air conditioning apparatus for vehicles has a refrigerant circuit including a condenser, a pressure reducing device (for example, an expansion valve), an evaporator, and a compressor. The compressor recently adopted is often a variable displacement compressor (particularly, a swash plate type variable displacement compressor) that is flexible to meet various air-conditioning requirements. Generally, a prior-art swash plate type variable displacement compressor varies its displacement by maintaining the pressure acting on an evaporator outlet (suction pressure Ps) at a predetermined target value (target suction pressure). That is, the compressor has a displacement control valve that controls the compressor displacement in a feedback manner in accordance with the suction pressure Ps, which serves as a reference indicator, such that the displacement corresponds to the cooling load of the compressor. More specifically, a pressure sensitive member, such as a bellows or a diaphragm, detects the suction pressure Ps. The movement of the pressure sensitive member positions a valve body to adjust the opening size of the control valve. This varies the pressure (crank pressure Pc) in a swash plate chamber (crank chamber) to alter an inclination angle of the swash plate. That is, the piston stroke is varied in accordance with the inclination angle of the swash plate, which is controlled in a range from a minimum inclination angle xcex8min to a maximum inclination angle xcex8max. The compressor displacement is thus adjusted as necessary in a range from minimum to a maximum.
However, a detailed operation analysis regarding this swash plate type variable displacement compressor indicates that the compressor is not capable of ensuring a uniform operational efficiency for the entire range in which the displacement is varied. The operational efficiency of the compressor (or an air conditioning apparatus including the compressor) is represented by a coefficient of performance (COP) and is indicated by the following equation: COP=Q/L. In the equation, Q indicates refrigerating performance (heat absorbing performance of the evaporator), and L indicates the power supplied to the compressor (workload of the compressor). As the COP increases, the operational efficiency of the compressor increases.
FIG. 7 is a graph in which refrigerating performance ratio (Q/Q0) is plotted along the horizontal axis (X-axis) and power ratio (L/L0) is plotted along the vertical axis (Y-axis). Q0 indicates a maximum refrigerating performance. If the equation Q=Q0 is satisfied, the refrigerating performance ratio Q/Q0 is 100%. In the same manner, L0 indicates a maximum power supplied to the compressor. If the equation L=L0 is satisfied, the power ratio L/L0 is 100%. In the graph, a diagonal broken line extends from the origin (0, 0) to a point indicating a maximum performance: (L0/L0, Q0/Q0)=(1, 1). Along this diagonal straight line, the following equation is satisfied: Q/Q0=L/L0. Based on this equation, the following equation is obtained: Q0/L0=Q/L=COP. In other words, the area located above the diagonal straight line in the graph of FIG. 7 indicates a decrease in the COP, as compared to the maximum performance COP (COP=Q0/L0). In contrast, the area located below the diagonal straight line in the graph indicates an increased COP, as compared to the maximum performance COP (COP=Q0/L0)
As shown in FIG. 7, the graph includes three curves. The curves indicate characteristics of the swash plate type variable displacement compressor operated under different conditions regarding the suction pressure Ps and the like. The conditions are varied among the curves. As indicated by the graph, each curve crosses the diagonal straight line at a point P (referred to as the xe2x80x9cpoints of divergencexe2x80x9d). In an area of the power ratio located above each point P, as viewed in the graph, corresponding sections of the curves are located below the diagonal line. These sections of the curves thus indicate a relative increase in the COP, as compared to the maximum performance COP. In contrast, in an area of the power ratio located downward with respect to the points P, corresponding sections of the curve are located above the diagonal line. These sections of the curves thus indicate a relative reduction of the COP, as compared to the maximum performance COP. The power L supplied to the compressor increases as the inclination angle of the swash plate, or the compressor displacement, increases. Accordingly, as is clear from the graph of FIG. 7, the operational efficiency of the compressor decreases if the power supplied to the compressor is smaller than the value corresponding to the point P, or if the displacement is relatively small. Further, if the power supplied to the compressor is greater than the value corresponding to the point P, or the displacement is relatively large, the operational efficiency of the compressor is improved.
It is assumed that the lower operational efficiency during the relatively small displacement operation is caused by the following: (a) a reduced piston stroke decreases the sealing effect between the outer surface of each piston and the inner wall of the corresponding cylinder bore, thus increasing gas leakage from the cylinder bore to the crank chamber; (b) a greater amount of gas must be supplied to the crank chamber from the discharge chamber to maintain the crank pressure Pc at a relatively high level during lower displacement operation, and the amount of waste gas is increased; and (c) the proportion of mechanical power loss caused by friction for moving movable parts including the swash plate is increased during lower displacement operation.
As described, even though the compressor is capable of controlling of the displacement continuously in the entire range from minimum to maximum, this control is not necessarily advantageous regarding the operational efficiency of the compressor.
Accordingly, it is an objective of the present invention to provide a variable displacement compressor, the operational efficiency of which is improved by avoiding operation under conditions that reduce operational efficiency, and an air conditioning apparatus employing this variable displacement compressor.
To achieve the above objective, the present invention is a variable displacement compressor that varies the displacement in a variation range including a minimum displacement and a maximum displacement. The compressor includes an acquiring device for acquiring a target value used for controlling the compressor displacement, a switching device, which compares the target value with a predetermined reference value and switches an operational mode in accordance with a result from the comparison such that the displacement corresponding to the target value achieves a coefficient of performance equal to or greater than a predetermined level, and an actuator for varying the displacement in accordance with an instruction from at least the switching device.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.